Patients with kidney failure or partial kidney failure typically undergo hemodialysis treatment, often at a hemodialysis treatment center or clinic. When healthy, kidneys maintain the body's internal equilibrium of water and minerals (e.g., sodium, potassium, chloride, calcium, phosphorous, magnesium, and sulfate). The kidneys also function as part of the endocrine system to produce the hormone erythropoietin as well as other hormones. Hemodialysis is an imperfect treatment to replace kidney function, in part, because it does not correct the endocrine functions of the kidney.
In hemodialysis, blood is taken from a patient through an intake needle (or catheter) which draws blood from an artery located in a specific accepted access location (arm, thigh, subclavian, etc.). The drawn blood is pumped through extracorporeal tubing via a peristaltic pump, and then through a dialyzer which removes unwanted toxins such as blood urea, nitrogen, potassium, and excess water from the blood. As the blood passes through the dialyzer, it travels in straw-like tubes which serve as semi-permeable membrane passageways for the uncleaned blood. Fresh dialysate liquid, which is a solution of chemicals and water, flows through the dialyzer in the direction opposite the blood flow. As the dialysate flows through the dialyzer, it surrounds the straw-like membranes in the dialyzer. The fresh dialysate collects excess impurities passing through the straw-like tubes by diffusion, and also collects excess water through an ultrafiltration process due to a pressure drop across the membranes. The used dialysate exits the dialyzer with the excess fluids and toxins via an output tube, thus cleansing the blood flowing through the dialyzer. The dialyzed blood then flows out of the dialyzer via tubing and a needle (or catheter) back into the patient. Sometimes, a heparin drip or pump is provided along the extracorporeal blood flow loop in order to prevent clotting during the hemodialysis process. Several liters of excess fluid can be removed during a typical multi-hour treatment session. In the U.S., a chronic patient will normally undergo hemodialysis treatment in a dialysis center three times per week, either on Monday-Wednesday-Friday schedule or a Tuesday-Thursday-Saturday schedule.
Hemodialysis has an acute impact on the fluid balance of the body due in part to the rapid change in circulating blood volume. When the fluid removal rate is more rapid than the plasma refilling rate of the body, intravascular blood volume decreases. The resulting imbalance has been linked to complications such as hypotension, loss of consciousness, headaches, vomiting, dizziness and cramps experienced by the patient, both during and after dialysis treatments. Continuous quantitative measurement of parameters relating to the circulating blood volume (in real-time) during hemodialysis reduces the chance of dialysis-induced hypotension, and otherwise optimizes dialysis therapy regimes by controlling fluid balance and aiding in achieving the appropriate dry weight for the patient.
In the art, it is known that, during a hemodialysis treatment session, the change in hematocrit value is inversely proportional to the change in blood volume. For example, see U.S. Pat. No. 5,351,686 entitled “Disposable Extracorporeal Conduit For Blood Constituent Monitoring”, assigned to the assignee of the present application and issuing on Oct. 4, 1994. The hematocrit value is the percentage of blood volume occupied by red blood cells. Since the number of red blood cells remains substantially constant during dialysis treatments that do not include bleeding or transfusions, the hematocrit will change only as a result of changing blood volume. Therefore, the change in blood volume during hemodialysis can be monitored by measuring hematocrit during the hemodialysis session.
There are several techniques to monitor hematocrit, although many are not practical for real-time monitoring during hemodialysis. The most common technique is a manual method in which a syringe is used to extract blood from the patient. The extracted blood is then put into a capillary device which is placed in a microcentrifuge. Because of the blood draw, this manual process is inadequate for monitoring real-time changes in blood volume during a hemodialysis treatment session. It has also been found to be somewhat inaccurate in practice. In addition, even if blood is drawn only a few times per week each blood draw lowers the patient iron level and induces further anemia. Therefore, in practice, hemodialysis patients normally have manual blood work done to determine hematocrit, as well as other blood abnormalities, about once a month.
Another device for measuring hematocrit is called a cell counter or Coulter counter. In a cell counter, a metered volume of blood is tested. The red blood cells are literally counted as they drop through a small diameter pipette within the cell counter. The mean cell volume of the red blood cells is measured via an electrical current that passes through a designated area of the pipette. The size of the blood cell correlates to the amount of electrical current passed. The volume of red blood cells is found by multiplying the red blood cell count times the mean cell volume. The hematocrit is determined by dividing the calculated red blood cell volume over the total volume of the sample. This method is assumed in the art to be more accurate than the microcentrifuge method. However, this method again requires that a blood sample be taken from the patient.
Another way to monitor hematocrit is to use a non-invasive, real-time optical blood monitor. The assignee of the present application manufactures such an optical blood monitoring system marketed under the name CRIT-LINE®. The CRIT-LINE® system continuously monitors the change in hematocrit over a dialysis session, and uses this information to calculate and display accurate percent blood volume change. It can also determine and display oxygen saturation levels and hemoglobin levels. The dynamic display of blood data during a hemodialysis session, including the display of change in blood volume, is quite helpful to attending staff administering the treatment to the patient.
To use the CRIT-LINE® system, a sterile, single-use blood chamber is attached, prior to hemodialysis treatment, inline in extracorporeal tubing on the arterial side of the dialyzer. The blood chamber provides a viewing point for optical sensors during the hemodialysis procedure. Multiple wavelengths of light are directed through the blood chamber and the patient's blood flowing therethrough, and photo detectors detect the resulting intensity of each wavelength. The preferred wavelengths are about 810 nanometers, which is substantially isobectic for red blood cells containing hemoglobin, and about 1300 nanometers, which is substantially isobectic for water. A ratiometric technique implemented in the CRIT-LINE® controller, as substantially disclosed in U.S. Pat. No. 5,372,136 entitled “System And Method For Non-invasive Hematocrit Monitoring”, which issued on Dec. 13, 1999 and is also assigned to the assignee of the present application, uses this information to calculate the patient's hematocrit value (HCT) in real-time, which as mentioned is the percentage of blood volume that is occupied by red blood cells. The CRIT-LINE® monitor provides an absolute measurement of hematocrit in real-time that is independent of other blood analytes. One of the advantages of monitoring the patient's blood non-invasively is that it is not necessary to take a blood draw, and therefore the anemic condition is not exacerbated.
The CRIT-LINE® monitor estimates patient hemoglobin levels (Hgb) from the measured hematocrit value. Hemoglobin levels are expressed as the amount of hemoglobin in grams (gm) per deciliter (dl) of whole blood. The most common direct method of measuring hemoglobin requires the extraction of a blood sample from the patient, and then treating the blood with a lysing agent in the laboratory in order to rupture the red blood cell membranes and release the hemoglobin into solution so that its concentration can be measured. Obviously, this technique cannot be implemented in real-time in the CRIT-LINE® monitor. Rather, the CRIT-LINE® system estimates real-time hemoglobin level based on the measured real-time hematocrit level (e.g. HCT=2.941 Hgb).
Oxygen saturation (SAT) measures the percentage of hemoglobin binding sites occupied by oxygen. Hematocrit independent oxygen saturation is measured in the CRIT-LINE® monitor using a photo emitter having a wavelength of about 660 nanometers. The photo detector monitors the intensity of 660 nm light after it passes through the blood chamber and the blood flowing through the blood chamber. A ratiometric model using the intensity of detected light at 660 nm and at substantially 810 nm is used to determine the real-time oxygen saturation level in the CRIT-LINE® monitor.
The CRIT-LINE® monitor is thus able to non-invasively monitor in real-time during hemodialysis the patient's hematocrit (HCT), change in blood volume (BVΔ), oxygen saturation (SAT), and calculated hemoglobin (Hgb) levels.
Hemodialysis centers normally include several hemodialysis systems so that multiple patients can be treated contemporaneously. In many centers, a dedicated CRIT-LINE® monitor is used in connection with each individual hemodialysis system. The display on the CRIT-LINE® monitor (which is normally located next to the patient) helps the attending nurses insure that hematocrit, oxygen saturation, and blood volume levels, as well as calculated hemoglobin levels, remain within the accepted tolerances for the treated patient. The patient data, such as hematocrit, oxygen saturation, and calculated hemoglobin values, is often downloaded to a host computer for patient records.
As kidney function decreases, one of the side effects is that erythropoietin synthesis decreases, which can potentially lead to anemia, causing fatigue in the patient. Hemoglobin variability is common in patients with End Stage Renal Disease (ESRD) on hemodialysis. Erythropoiesis stimulating agents (e.g., recombinant erythropoietin), commonly known as ESAs, are pharmaceutically produced and administered by physicians to hemodialysis patients in order to manage anemia when present. Recombinant ESA can be administered either subcutaneously via syringe, or via a drip in the extracorporeal tubing of the hemodialysis loop, normally at the end of a hemodialysis treatment session. The purpose of administering ESA is to maintain the patient's hemoglobin levels within a healthy range. Under dosing ESA results in low hemoglobin levels. On the other hand, overuse of ESA can result in excessive cost, as well as undesirable side effects.